The invention relates generally to coated spheres and methods of making same. More particularly, the invention relates to the method of applying a protective coating to solder alloy spheres by a process of vapor deposition.
Electrical components such as resisters, capacitors, inductors, transistors, integrated circuits, chip carriers and the like are typically mounted on circuit boards and other electronic substrates by one of two methods that are well known in the electronics assembly industries. A first method includes mounting the components to a first side of a circuit board. Leads from the components extend through holes formed in the circuit board and are soldered on an opposing side of the board. A second method includes soldering components on the same side of the printed circuit board to which they are mounted. These latter components are said to be xe2x80x9csurface-mountedxe2x80x9d to circuit boards.
Surface mounting electronic components to circuit boards and other electronic substrates is a desirable method in that the method may be used to fabricate very small circuit structures. In addition, surface mounting lends itself well to process automation. In high-density electronic manufacturing processes, surface mountable microelectronic devices are bonded to a substrate by a solder reflow process. One type of surface mountable device, commonly referred to as a xe2x80x9cflip chip,xe2x80x9d comprises an integrated circuit device having numerous connecting leads that are attached to pads mounted on the underside of the device. With the use of flip chips, either the circuit board or the chip is provided with small bumps or balls of solder (hereinafter referred to as xe2x80x9cspheresxe2x80x9d or xe2x80x9csolder spheresxe2x80x9d) that are positioned in locations which correspond to the pads on the underside of the chip and on the surface of the circuit board. The solder spheres are formed prior to the reflow process by any of various prior art processes which include deposition through a mask, electroplating, pick-and-place, evaporation, sputtering, and screen printing.
The chip is mounted to the circuit board or other electronic substrate by (a) placing it in contact with the board such that the solder spheres become sandwiched between the pads on the board and the corresponding pads on the chip forming an assembly; (b) heating the assembly to a point at which the solder reflows; and (c) cooling the assembly. Upon cooling, the solder hardens, thereby mounting the flip chip to the surface of the circuit board.
Tolerances in devices using flip chip technology are critical, as the spacing between individual devices as well as the spacing between the chip and the circuit board is typically very small. For example, spacing of such chips from the surface of the board is typically in the range of 0.5 to 3.0 mil and is expected to approach micron spacing in the near future.
For example, electrical connections in BGA (ball grid array) packages are made by placing solder spheres of precisely controlled diameter and unblemished surface condition between circuit pads. Solder spheres are then heated above the liquidus temperature of the solder alloy, thereby melting the solder spheres, which wets and flows onto both contact pads, creating a mechanical and an electrical contact.
Commonly used solder alloys consist of relatively soft base metals such as aluminum or copper that can be easily damaged by mechanical agitation. Such damage may result in, for example, the formation of surface flat cracks and crevices; spalling off portions of the spheres as particles or flakes; loss of the bright reflective sphere surface; increased sphere electrical contact and bulk resistivity; and an exacerbation of base metal oxidation at the sphere surface.
Damage to the solder sphere surface may produce a range of consequences. For example, automated vision system assembly hardware will not be able to distinguish a solder sphere from the background if the sphere reflectivity has been diminished. Additionally, physical surface damage will hinder the ability of most automated BGA assembly hardware to pick and place individual spheres. Furthermore, the presence of extraneous particles on the solder spheres may impair the mechanical function of the BGA assembly hardware and cause low resistivity or electrical shorts between contact pads on the microelectronic package, and impact electrical performance once the BGA joints have been created. Importantly, excessive oxide present on the solder sphere surface can impair proper wetting and flow of solder spheres into contact pads as necessary to form an adequate mechanical joint and electrical connection.
In an effort to protect solder sphere surfaces from oxidation, the production of solder spheres coated with low melting point materials such as solder or flux has previously been disclosed in U.S. Pat. Nos. 5,872,400 and 5,736,074.
However, the need remains in the art to provide a rapid and efficient method of physically protecting solder alloy spheres from mechanical damage prior to use in surface mount techniques. The present invention provides a rapid and efficient method of coating solder alloy spheres to prevent mechanical surface damage and surface oxidation of alloy metals.
The invention provides a method for applying a chemical coating to surfaces of solder alloy spheres to ameliorate or eliminate mechanical damage due to contact and collision of the solder alloy spheres with other solder spheres and side walls of containers used for storing and transporting the solder alloy spheres. In addition, chemically coating surfaces of the solder alloy spheres ameliorates or eliminates oxidation of metal alloys comprising surfaces of the solder spheres.
The method of the invention includes the step of providing a first vapor-tight chamber into which a coating solution, formulated as described herein and comprising a volatile organic solvent and at least one solute, such as a low viscosity organic material and a surfactant. The method further includes the step of providing a second chamber containing a plurality of solder spheres and immersing the second chamber into the solution contained in the first vapor-tight chamber. The solder spheres are in fluid communication with the coating solution by a plurality of apertures or perforations formed in the second chamber. The solder spheres are immersed in the coating solution for a desired predetermined residence time. In one embodiment, after expiration of the residence time, the second chamber with the solder spheres contained therein is removed from the first vapor-tight chamber and placed in a second vapor-tight chamber. The second vapor-tight chamber is heated to a temperature above a boiling point of the solvent used in the coating solution by a heating device in order to vaporize any solvent remaining that did not adhere to surfaces of the solder spheres. In addition, vaporization of excess solvent may be accomplished by decreasing a pressure in the second vapor-tight chamber. After heating the second vapor-tight chamber for a desired predetermined time, the second chamber with the solder spheres contained therein is removed.
In one embodiment, the temperature of the second vapor-tight chamber may be controlled by a thermal sensor. In another embodiment, the second vapor-tight chamber may be additionally equipped with a condenser to condense excess solvent vapors and a collection device for collecting the condensed solvent vapors for reuse.
The coating solution may include the volatile organic solvent selected from the group consisting of acetone, isopropyl alcohol, denatured ethanol, n-propyl bromide, trichloroethylene, Genesolve 2000(trademark), Ensolv(trademark), Asahi AK-225(trademark) and Vaporedge 1000(trademark).
In addition, the coating solution contains the low viscosity organic material which may be selected from the group consisting of paraffin oil, mineral oil, isostearic acid, polyolefin oil, adipic acid, silicone oil, petroleum oil and tin, and any combination thereof. The low viscosity organic material is present at a concentration of from about 0.05 percent by weight to 5.0 percent by weight (wt. %).
The surfactant of the coating solution may be selected from the group consisting of simethicone, cyclomethicone, decamethylcyclopentasiloxane, and any combination thereof. The surfactant is present at a concentration of from about 0.01 wt. % to about 1.0 wt. %.
The coating solution may further include a solvent-soluble ultraviolet UV fluorescent dye known to those skilled in the art as a fluor. A coating solution containing a fluor leaves a UV fluorescent deposit on surfaces of the solder spheres that assists to optically locate the solder spheres. The UV fluorescent dye is present at a concentration of from about 0.01 to about 0.1 wt. %.
The coating solution may also include a solvent-soluble, polar or non-polar solder flux that minimizes or eliminates the need for a separate deposition of liquid flux or flux paste onto surfaces of the solder spheres during the reflow phase of surface mounting. The solder flux is present at a concentration of from about 05 to about 1.0 wt. %.
Other advantages and features of the invention will become more readily apparent from the following detailed description taken in connection with the appended claims and accompanying drawing.